Systems and methods for avoiding MRI-originated interference with concurrently used systems

ABSTRACT

MRI interference with a concurrently operated system may be reduced or corrected by subtracting the MRI interference from signals measured using the concurrently operated system. Various approaches for performing MRI of an anatomic region in conjunction with a radio-frequency-sensitive (RF-sensitive) measurement of the region using the concurrently operated system include the steps of simultaneously performing an MR scan sequence including MR pulses and the RF-sensitive measurements; recording the RF-sensitive measurements as they are made; detecting intervals during the MR scan sequence when an RF level is sufficient to interfere with the RF-sensitive measurements; and retaining only the RF-sensitive measurements performed outside the detected intervals.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 13/222,086,filed on Aug. 31, 2011, the entire disclosure of which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates, generally, to medical diagnosis andtreatment methods guided by magnetic resonance imaging (MRI), and, morespecifically, to approaches to minimizing MRI-induced interference.

BACKGROUND

Magnetic resonance imaging may be used in conjunction with ultrasoundfocusing in a variety of medical applications. Ultrasound penetrateswell through soft tissues and, due to its short wavelengths, can befocused to spots with dimensions of a few millimeters. As a consequenceof these properties, ultrasound can be and has been used for variousdiagnostic and therapeutic medical purposes, including ultrasoundimaging and non-invasive surgery. For example, focused ultrasound may beused to ablate diseased (e.g., cancerous) tissue without causingsignificant damage to surrounding healthy tissue. An ultrasound focusingsystem generally utilizes an acoustic transducer surface, or an array oftransducer surfaces, to generate an ultrasound beam. In transducerarrays, the individual surfaces, or “elements,” are typicallyindividually controllable—i.e., their vibration phases and/or amplitudescan be set independently of one another—allowing the beam to be steeredin a desired direction and focused at a desired distance. The ultrasoundsystem often also includes receiving elements, integrated into thetransducer array or provided in form of a separate detector, that helpmonitor the focused ultrasound treatment, primarily for safety purposes.For example, the receiving elements may serve to detect ultrasoundreflected off interfaces between the transducer and the target tissue,which may result from air bubbles on the skin that need to be removed toavoid skin burns. The receiving elements may also be used to detectcavitation in overheated tissues (i.e., the formation of cavities due tothe collapse of bubbles formed in the liquid of the tissue).

To visualize the target tissue and guide the ultrasound focus duringtherapy, magnetic resonance imaging may be used. In brief, MRI involvesplacing a subject, such as the patient, into a homogeneous staticmagnetic field, thus aligning the spins of hydrogen nuclei in thetissue. Then, by applying a radio-frequency (RF) electromagnetic pulseof the right frequency (the “resonance frequency”), the spins may beflipped, temporarily destroying the alignment and inducing a responsesignal. Different tissues produce different response signals, resultingin a contrast among theses tissues in MR images. Because the resonancefrequency and the frequency of the response signal depend on themagnetic field strength, the origin and frequency of the response signalcan be controlled by superposing magnetic gradient fields onto thehomogeneous field to render the field strength dependent on position. Byusing time-variable gradient fields, MRI “scans” of the tissue can beobtained. Many MRI protocols utilize time-dependent gradients in two orthree mutually perpendicular directions. The relative strengths andtiming of the gradient fields and RF pulses are specified in a pulsesequence and may be illustrated in a pulse sequence diagram.

Time-dependent magnetic field gradients may be exploited, in combinationwith the tissue dependence of the MRI response signal, to visualize, forexample, a brain tumor, and determine its location relative to thepatient's skull. An ultrasound transducer system, such as an array oftransducers attached to a housing, may then be placed on the patient'shead. The ultrasound transducer may include MR tracking coils or othermarkers that enable determining its position and orientation relative tothe target tissue in the MR image. Based on computations of the requiredtransducer element phases and amplitudes, the transducer array is thendriven so as to focus ultrasound into the tumor. Alternatively oradditionally, the ultrasound focus itself may be visualized, using atechnique such as thermal MRI or acoustic resonance force imaging(ARFI), and such measurement of the focus location may be used to adjustthe focus position. These methods are generally referred to asmagnetic-resonance-guided focusing of ultrasound (MRgFUS).

In addition, an MRI apparatus and an ultrasound imaging system may becombined to offer the strengths of both imaging modalities and therebyprovide novel insights into the morphology and function of normal anddiseased tissues. MRI is used widely for both diagnostic and therapeuticapplications because of its multi-planar imaging capability, highsignal-to-noise ratio, and sensitivity to subtle changes in soft tissuemorphology and function. Ultrasound imaging, on the other hand, hasadvantages including high temporal resolution, high sensitivity toacoustic scatters (such as calcifications and gas bubbles), excellentvisualization, and measurement of blood flow, low cost, and portability.Benefits of combining these complementary modalities have been shown inintraoperative neurosurgical applications and breast biopsy guidance. Byperforming imaging with both modalities simultaneously, a number ofissues such as spatial and temporal registration between data sets maybe simplified. In addition, measurements of unique physiologicalparameters can be made with each modality to fully characterize theorgan or tissue under evolution.

The simultaneous operation of ultrasound and MRI apparatus, however, canlead to undesired interferences. For example, MRI is very sensitive toRF noise generated by the focused ultrasound system (see, e.g., U.S.Pat. No. 6,735,461). Conversely, focused ultrasound procedures ofteninvolve RF-sensitive operations (such as the ultrasound detection thatmay accompany treatment with focused ultrasound) that are easilydisturbed by RF excitation signals and/or time-varying field gradientgenerated by the MRI system. Prior-art approaches to avoiding suchinterference typically involve shielding. Shielding the ultrasoundsystem from interfering MR signals typically requires covering orsurrounding the whole transducer and associated cables in metallicshield. In some systems, however, acoustic constraints prevent completeencapsulation of the ultrasound-receiving elements, resulting inpenetration of, e.g., the front layer of a receiver and/or the cables bysome amount of RF noise. Accordingly, there is a need for alternativeapproaches in MRgFUS applications to minimize or avoid interferencesbetween the two systems.

SUMMARY

Embodiments of the present invention provide various approaches tosimultaneously operating an MRI apparatus for imaging an anatomic regionand a system for concurrently performing RF-sensitive operations withoutor with reduced MRI interference. The concurrently operated system maybe any system or sensor that has diagnostic and therapeuticapplications. For example, it may be a treatment system (such as aphased-array ultrasound transducer system), an imaging system (such asan ultrasound imaging probe) or a signal-detection sensor (such as anultrasonic cavitation sensor). In various embodiments, the MRinterference originating from excitation of the MRI apparatus is firstmeasured by the signal-detection sensor and/or detection channel(s)associated with the concurrently operated system when the latter systemis idling—i.e., inactive or not actively transmitting energy (e.g.,acoustic energy), while the MRI apparatus is normally operated (toperform “cold” scans, i.e., scans that are not concurrent withtreatment, ultrasound imaging, or cavitation detections). Because MRinterference is typically an additive, stationary noise with respect tothe desired signal, signal(s) measured by the concurrently operatedsystem when both it and an MRI apparatus are simultaneously active(i.e., performing “hot” scans) can be corrected by subtracting the MRinterference measured during cold scans from the measured signal(s)obtained during hot scans. Accordingly, embodiments of the presentinvention correct signals measured by the concurrently operated systemso that they are not significantly distorted by MR interference; this isadvantageous in that it generally eliminates the need for shielding theRF signals as required by the prior art.

In some embodiments, a Fast Fourier Transform (FFT) approach is used toconvert the measured RF-sensitive signal(s) and MR interference to thefrequency domain. Subtraction of the MR interference from the measuredsignal(s) can then be performed on the converted signals at eachfrequency in the spectrum. This is particularly useful when the measuredsignal(s) of the concurrently operated system are insensitive to thephase measurements (e.g., cavitation measurements) and can thereby avoidthe need to accurately synchronize operations of the MRI apparatus andthe concurrently operated system.

If, during operation, the concurrently operated system is at leastpartially synchronized with the MRI scanning (i.e., synchronized onlywithin one or more time windows), the signal(s) measured by theconcurrently operated system may be corrected by identifying the MRinterference within the time window(s) and subtracting the MRinterference therefrom. In some embodiments, the MR interference ispseudo-stationary for each repetition time (or time to repeat (TR));accordingly, the MR interference obtained during the cold scans can bereduced to a single set of data. The single set of MR interference datamay correspond to, for example, a single cold scan TR, an average of MRinterference recorded during multiple cold scan TRs, or a maximum MRinterference in the multiple cold scan TRs. This approach can reducecomplexity when processing a massive data volume representing MRinterference and signal(s) acquired during simultaneous operation of theMRI apparatus and the RF-sensitive system.

Accordingly, in one aspect, the invention pertains to a method ofperforming magnetic resonance (MR) imaging of an anatomic region inconjunction with a radio-frequency-sensitive (RF-sensitive) measurementof the region using a concurrently operated system (such as aphased-array ultrasound transducer system, an ultrasound imaging probe,and/or a cavitation sensor). In various embodiments, the method includesthe steps of: with the concurrently operated system idle, performing anMR scan sequence including MR pulses; detecting intervals during the MRscan sequence when an RF level is sufficient to interfere with theRF-sensitive measurements; storing data indicative of a temporal extentof the MR scan sequence and the detected intervals thereduring; andbased at least in part on the stored data, simultaneously performing thescan sequence and operating the concurrently operated system butobtaining the RF-sensitive measurements only during times notcorresponding to the recorded intervals. The detecting step may beperformed by the concurrently operated system and/or one or morededicated sensors outside the concurrently operated system. The MR scansequence may be performed periodically and the RF-sensitive measurementsmay be performed at least partially in synchronization with at least oneof the MR scan sequence.

In another aspect, the invention relates to a system for performing MRimaging of an anatomic region in conjunction with a RF-sensitivemeasurement of the region. In various embodiments, the system includesan MR imaging apparatus for imaging the anatomic region; a concurrentlyoperated system for performing the RF-sensitive measurement; and acontroller in communication with the MR imaging apparatus and theconcurrently operated system. In one implementation, the controller isconfigured to: with the concurrently operated system idle, perform an MRscan sequence including MR pulses; determine intervals during the MRscan sequence when an RF level is sufficient to interfere with theRF-sensitive measurements; store data indicative of a temporal extent ofthe MR scan sequence and the detected intervals thereduring; based atleast in part on the stored data, simultaneously perform the scansequence and operate the concurrently operated system but obtain theRF-sensitive measurements only during times not corresponding to therecorded intervals. The determination of the intervals may be performedby the concurrently operated system and/or one or more dedicated sensorsoutside the concurrently operated system. In various embodiments, thecontroller is further configured to: (i) perform the MR scan sequenceperiodically; and (ii) perform the RF-sensitive measurements at leastpartially in synchronization with one or more MR scan sequences.

Another aspect of the invention relates to a method of performing MRimaging of an anatomic region in conjunction with a RF-sensitivemeasurement of the region using a concurrently operated system. Invarious embodiments, the method includes the steps of: simultaneouslyperforming an MR scan sequence including MR pulses and the RF-sensitivemeasurements; recording the RF-sensitive measurements as they are made;detecting intervals during the MR scan sequence when an RF level issufficient to interfere with the RF-sensitive measurements; andretaining only the RF-sensitive measurements performed outside thedetected intervals. The detecting step may be performed by theconcurrently operated system and/or by one or more dedicated sensorsoutside the concurrently operated system.

In yet another aspect, the invention pertains to a method of performingMR imaging of an anatomic region in conjunction with a RF-sensitivemeasurement of the region using a concurrently operated system. Invarious embodiments, the method includes the steps of: simultaneouslyperforming an MR scan sequence including MR pulses and obtaining theRF-sensitive measurements; recording the RF-sensitive measurements asthey are made; detecting intervals during the MR scan sequence when anRF level is sufficient to interfere with the RF-sensitive measurementsand recording the detected RF levels during the intervals; and adjustingthe recorded RF-sensitive measurements that were obtained during thedetected intervals based on the recorded RF levels. The detecting stepmay be performed by the concurrently operated system and/or by one ormore dedicated sensors outside the concurrently operated system.

Still another aspect of the invention relates to a system for performingMR imaging of an anatomic region in conjunction with a RF-sensitivemeasurement of the region. In various embodiments, the system includes:an MR imaging apparatus for imaging the anatomic region; a concurrentlyoperated system for performing the RF-sensitive measurement; and acontroller in communication with the MR imaging apparatus andconcurrently operated system. In one implementation, the controller isconfigured to: simultaneously perform an MR scan sequence including MRpulses and the RF-sensitive measurements; record the RF-sensitivemeasurements as they are made; determine intervals during the MR scansequence when an RF level is sufficient to interfere with theRF-sensitive measurements; and retain only the RF-sensitive measurementsperformed outside the detected intervals. The determination of theintervals may be performed by the concurrently operated system and/or byone or more dedicated sensors outside the concurrently operated system.

In another aspect, the invention relates to a system for performing MRimaging of an anatomic region in conjunction with a RF-sensitivemeasurement of the region. In some embodiments, the system includes: anMR imaging apparatus for imaging the anatomic region; a concurrentlyoperated system for performing the RF-sensitive measurement; and acontroller in communication with the MR imaging apparatus andconcurrently operated system. In various embodiments, the controller isconfigured to: simultaneously perform an MR scan sequence including MRpulses and obtaining the RF-sensitive measurements; record theRF-sensitive measurements as they are made; determine intervals duringthe MR scan sequence when an RF level is sufficient to interfere withthe RF-sensitive measurements and record the detected RF levels duringthe intervals; and adjust the recorded RF-sensitive measurements thatwere obtained during the detected intervals based on the recorded RFlevels. The determination of the intervals may be performed by theconcurrently operated system and/or by one or more dedicated sensorsoutside the concurrently operated system.

In still another aspect, the invention relates to a method of performingMR imaging of an anatomic region in conjunction with a RF-sensitivemeasurement of the region using a concurrently operated system. Invarious embodiments, the method includes the steps of: with theconcurrently operated system idle, performing an MR scan sequenceincluding MR pulses; detecting intervals during the MR scan sequencewhen a level of RF noise is sufficient to interfere with theRF-sensitive measurements; detecting a parameter of the RF noise duringthe intervals and recording the detected parameter; storing dataindicative of a temporal extent of the MR scan sequence and the detectedintervals and parameter of the RF noise thereduring; simultaneouslyperforming an MR scan sequence including MR pulses and obtaining theRF-sensitive measurements; recording the RF-sensitive measurements asthey are made; and adjusting the recorded RF-sensitive measurements thatwere obtained during the detected intervals based on the recorded RFlevels.

The adjustment may correspond to subtracting the recorded RF levels fromthe recorded RF-sensitive measurements that were obtained during thedetected intervals. In addition, the detected parameter may be aspectrum of the RF noise and the subtracting step may include, for eachRF-sensitive measurement, obtaining a spectrum of a signal correspondingto the measurement and subtracting, at each frequency in the spectra, amagnitude of the RF noise from a magnitude of the signal correspondingto the measurement. The spectra may be obtained from time-domain signalmeasurements using a Fast Fourier Transform. In one embodiment, thedetected parameter is a maximum level of the RF noise during an intervaland the subtracting step includes subtracting the maximum RF noise fromthe corresponding recorded RF-sensitive measurement. In anotherembodiment, the detected parameter is an average level of the RF noiseand the subtracting step includes subtracting the average RF noise fromeach recorded RF-sensitive measurement.

In some embodiments, the MR scan sequence is performed periodically andthe RF-sensitive measurements are performed at least partially insynchronization with one or more MR scan sequences. Additionally, themethod further includes determining time windows of the RF-sensitivemeasurements and identifying the RF noise within the determined timewindows; adjustment of the recorded RF-sensitive measurementscorresponds to subtracting the identified RF noise from the recordedRF-sensitive measurements that were obtained during the detected timewindows.

In another aspect, the invention pertains to a system for performing MRimaging of an anatomic region in conjunction with a RF-sensitivemeasurement of the region. In various embodiments, the system includes:an MR imaging apparatus for imaging the anatomic region; a concurrentlyoperated system for performing the RF-sensitive measurement; and acontroller in communication with the MR imaging apparatus andconcurrently operated system. In one implementation, the controller isconfigured to: with the concurrently operated system idle, perform an MRscan sequence including MR pulses; determine intervals during the MRscan sequence when a level of RF noise is sufficient to interfere withthe RF-sensitive measurements; determine a parameter of the RF noiseduring the intervals and record the detected parameter; store dataindicative of a temporal extent of the MR scan sequence and the detectedintervals and parameter of the RF noise thereduring; simultaneouslyperform an MR scan sequence including MR pulses and obtain theRF-sensitive measurements; record the RF-sensitive measurements as theyare made; and adjust the recorded RF-sensitive measurements that wereobtained during the detected intervals based on the recorded RF levels.

The adjustment may correspond to subtracting the recorded RF levels fromthe recorded RF-sensitive measurements that were obtained during thedetected intervals. In addition, the detected parameter may be aspectrum of the RF noise and the controller may be further configuredto, for each RF-sensitive measurement, obtain a spectrum of a signalcorresponding to the measurement and subtract, at each frequency in thespectra, a magnitude of the RF noise from a magnitude of the signalcorresponding to the measurement. The spectra may be obtained fromtime-domain signal measurements using a Fast Fourier Transform. In oneembodiment, the detected parameter is a maximum level of the RF noiseduring an interval and the controller is further configured to subtractthe maximum RF noise from the corresponding recorded RF-sensitivemeasurement. In another embodiment, the detected parameter is an averagelevel of the RF noise and the controller is further configured tosubtract the average RF noise from each recorded RF-sensitivemeasurement.

In various embodiments, the controller is further configured to: (i)perform the MR scan sequence periodically; and (ii) perform theRF-sensitive measurements at least partially in synchronization with oneor more MR scan sequences. Additionally, the controller is furtherconfigured to: determine time windows of the RF-sensitive measurements;and identify the RF noise within the determined time windows; adjustmentof the recorded RF-sensitive measurements corresponds to subtracting theidentified RF noise from the recorded RF-sensitive measurements thatwere obtained during the detected time windows.

As used herein, the terms “approximately,” “roughly,” and“substantially” mean ±10%, and in some embodiments, ±5%. Referencethroughout this specification to “one example,” “an example,” “oneembodiment,” or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the example isincluded in at least one example of the present technology. Thus, theoccurrences of the phrases “in one example,” “in an example,” “oneembodiment,” or “an embodiment” in various places throughout thisspecification are not necessarily all referring to the same example.Furthermore, the particular features, structures, routines, steps, orcharacteristics may be combined in any suitable manner in one or moreexamples of the technology. The headings provided herein are forconvenience only and are not intended to limit or interpret the scope ormeaning of the claimed technology.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, with an emphasis instead generally being placedupon illustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A schematically depicts an exemplary MRI system in accordance withvarious embodiments of the current invention;

FIG. 1B schematically depicts an exemplary system (e.g., focusedultrasound system) concurrently operated with the MRI system inaccordance with various embodiments of the current invention;

FIG. 2 schematically illustrates interactions between an MRI apparatusand a concurrently operated system in accordance with variousembodiments of the current invention;

FIG. 3A is a pulse sequence diagram illustrating an exemplary MRIprotocol as well as detection periods carried out by a concurrentlyoperated system in accordance with various embodiments of the currentinvention;

FIG. 3B depicts MR gradient-active periods determined based on asummation of the active-gradient times detected by some individual MRreceivers/sensors in accordance with various embodiments of the currentinvention; and

FIGS. 4A-4E depict various approaches for measuring RF-sensitive signalswith reduced or eliminated MRI interference in accordance with variousembodiments of the current invention.

DETAILED DESCRIPTION

FIG. 1A illustrates an exemplary MRI apparatus or system 102. Theapparatus 102 may include a cylindrical electromagnet 104, whichgenerates the requisite static magnetic field within a bore 106 of theelectromagnet 104. During medical procedures, a patient is placed insidethe bore 106 on a movable support table 108. A region of interest 110within the patient (e.g., the patient's head) may be positioned withinan imaging region 112 wherein the electromagnet 104 generates asubstantially homogeneous field. A set of cylindrical magnet fieldgradient coils 113 may also be provided within the bore 106 andsurrounding the patient. The gradient coils 113 generate magnetic fieldgradients of predetermined magnitudes, at predetermined times, and inthree mutually orthogonal directions. With the field gradients,different spatial locations can be associated with different precessionfrequencies, thereby giving an MR image its spatial resolution. An RFtransmitter coil 114 surrounding the imaging region 112 emits RF pulsesinto the imaging region 112 to cause the patient's tissues to emitmagnetic-resonance (MR) response signals. Raw MR response signals aresensed by the RF coil 114 and passed to an MR controller 116 that thencomputes an MR image, which may be displayed to the user. Alternatively,separate MR transmitter and receiver coils may be used. Images acquiredusing the MRI apparatus 102 may provide radiologists and physicians witha visual contrast between different tissues and detailed internal viewsof a patient's anatomy that cannot be visualized with conventional x-raytechnology.

The MRI controller 116 may control the pulse sequence, i.e., therelative timing and strengths of the magnetic field gradients and the RFexcitation pulses and response detection periods. The MR responsesignals are amplified, conditioned, and digitized into raw data using animage processing system, and further transformed into arrays of imagedata by methods known to those of ordinary skill in the art. Based onthe image data, a treatment region (e.g., a tumor) is identified. Theimage processing system may be part of the MRI controller 116, or may bea separate device (e.g., a general-purpose computer containing imageprocessing software) in communication with the MRI controller 116. Insome embodiments, one or more ultrasound systems 120 or one or moresensors 122 are displaced within the bore 106 of the MRI apparatus 102as further described below.

FIG. 1B illustrates an exemplary system 150, such as an ultrasoundsystem, concurrently operated with the MRI system 102 in accordance withsome embodiments of the present invention, although alternativeconcurrently operated systems with similar or different functionalitythat may interfere with the MRI system 102 are also within the scope ofthe invention. As shown, the ultrasound system includes a plurality ofultrasound transducer elements 152, which are arranged in an array 153at the surface of a housing 154. The array may comprise a single row ora matrix of transducer elements 152. In alternative embodiments, thetransducer elements 152 may be arranged without coordination, i.e., theyneed not be spaced regularly or arranged in a regular pattern. The arraymay have a curved (e.g., spherical or parabolic) shape, as illustrated,or may include one or more planar or otherwise shaped sections. Itsdimensions may vary, depending on the application, between millimetersand tens of centimeters. The transducer elements 152 may bepiezoelectric ceramic elements. Piezo-composite materials, or generallyany materials capable of converting electrical energy to acousticenergy, may also be used. To damp the mechanical coupling between thetransducer elements 152, they may be mounted on the housing 154 usingsilicone rubber or any other suitable damping material.

The transducer elements 152 are separately controllable, i.e., they areeach capable of emitting ultrasound waves at amplitudes and/or phasesthat are independent of the amplitudes and/or phases of the othertransducers. A transducer controller 156 serves to drive the transducerelements 152. For n transducer elements, the transducer controller 156may contain n control circuits each comprising an amplifier and a phasedelay circuit, each control circuit driving one of the transducerelements. The transducer controller 156 may split an RF input signal,typically in the range from 0.1 MHz to 4 MHz, into n channels for the ncontrol circuit. It may be configured to drive the individual transducerelements 152 of the array at the same frequency, but at different phasesand different amplitudes so that they collectively produce a focusedultrasound beam. The transducer controller 156 desirably providescomputational functionality, which may be implemented in software,hardware, firmware, hardwiring, or any combination thereof, to computethe required phases and amplitudes for a desired focus location. Ingeneral, the transducer controller 156 may include several separableapparatus, such as a frequency generator, a beamformer containing theamplifier and phase delay circuitry, and a computer (e.g., ageneral-purpose computer) performing the computations and communicatingthe phases and amplitudes for the individual transducer elements 152 tothe beamformer. Such systems are readily available or can be implementedwithout undue experimentation.

To perform ultrasound imaging, the transducer controller 156 drives thetransducer elements 152 to transmit acoustic signals into a region beingimaged and to receive reflected signals from various structures andorgans within the patient's body. By appropriately delaying the pulsesapplied to each transducer element 152, a focused ultrasound beam can betransmitted along a desired scan line. Acoustic signals reflected from agiven point within the patient's body are received by the transducerelements 152 at different times. The transducer elements can thenconvert the received acoustic signals to electrical signals which aresupplied to the beamformer. The delayed signals from each transducerelement 152 are summed by the beamformer to provide a scanner signalthat is a representation of the reflected energy level along a givenscan line. This process is repeated for multiple scan lines to providesignals for generating an image of the prescribed region of thepatient's body. Typically, the scan pattern is a sector scan, whereinthe scan lines originate at the center of the ultrasound transducer andare directed at different angles. A linear, curvilinear or any otherscan pattern can also be utilized.

In various embodiments, during a focused ultrasound procedure, small gasbubbles, or “micro-bubbles,” are generated in the liquid contained inthe tissue due to the stress resulting from negative pressure producedby the propagating ultrasonic waves and/or from when the heated liquidruptures and is filled with gas/vapor. On one hand, the micro-bubbleshave a positive treatment effect by generating higher harmonicfrequencies of the original wave energy, thereby increasing theabsorption of energy in the tissue. On the other hand, the reaction oftissue containing a higher relative percentage of micro-bubbles to thecontinued application of the ultrasound energy is non-linear anddifficult to predict. For example, the micro-bubbles may collapse due tothe applied stress from an acoustic field. This mechanism, called“cavitation,” may cause extensive tissue damage beyond that targeted.Accordingly, to monitor the micro-bubbles in the target tissue when theacoustic waves are applied, in various embodiments, the concurrentlyoperated system 150 includes one or more ultrasonic cavitation sensors158. The cavitation sensor(s) 158 detects acoustic radiation that isemitted by the micro-bubbles due to a change of their volumes whendriven by the external applied acoustic field. By analyzing the spectralcharacteristics of the detected acoustic radiation, detailed informationregarding the dynamics of the cavitation process can be obtained.Because the acoustic signals emitted by the micro-bubbles are in an RFrange, they are easily disturbed by RF excitation signals and/ortime-varying field gradients generated by the MRI system 102.Accordingly, it is also critical to eliminate/avoid MR interference whendetecting the cavitation signals.

The concurrently operated system 150, such as the ultrasound systemand/or cavitation sensor(s), may be disposed within the bore 106 of theMRI apparatus 102 or placed in the vicinity of the MRI apparatus 102.For example, multiple cavitation sensors 158 may be provided to surroundthe imaging region 112. To aid in determining the relative position ofthe concurrently operated system 150 and MRI apparatus 102, theconcurrently operated system 150 may further include MR trackers 160associated therewith, arranged at a fixed position and orientationrelative to the concurrently operated system 150. The trackers 160 may,for example, be incorporated into or attached to the concurrentlyoperated system housing. If the relative positions and orientations ofthe MR trackers 160 and concurrently operated system 150 are known, MRscans of the MR trackers 160 implicitly reveal the location of theconcurrently operated system 150 in MRI coordinates, i.e., in thecoordinate system of the MRI apparatus 102.

As depicted in FIGS. 1A and 1B, a combined system including the MRIapparatus 102 and concurrently operated system 150 may be capable ofimaging the anatomic region of interest and detect ultrasound signals;the combined system may serve to monitor the application of ultrasoundfor treatment and/or safety purposes. For example, ultrasoundreflections off tissue interfaces that intersect the ultrasound beampath may be analyzed to ensure, if necessary by adjustment of thetreatment protocol, that such interfaces are not inadvertentlyoverheated. Further, measurements of the received cavitation spectrummay be used to detect cavitation resulting from the interaction ofultrasound energy with water-containing tissue. In addition, thevisualization of the tissue and target may be supplemented by ultrasoundimaging, for example, to facilitate tracking a moving target. Ultrasounddetection may be accomplished with the ultrasound transducer array 153.For example, treatment and imaging periods may be interleaved, or acontiguous portion of the array 153 or discontiguous subset oftransducer elements 152 may be dedicated to imaging while the remainderof the array 153 focuses ultrasound for treatment purposes.Alternatively, a separate ultrasound receiver 172, which may be, e.g., asimple ultrasound probe or array of elements, may be provided. Theseparate receiver 172 may be placed in the vicinity of the ultrasoundtransducer array 153, or integrated into its housing 154. In addition,the receiver 172 may be disposed within the bore 106 of the MRIapparatus 102 or placed in the vicinity thereof.

FIG. 2 schematically illustrates the interaction between an MRIapparatus 200 and a concurrently operated system 202, such as aphased-array ultrasound transducer system 204, an ultrasound imagingprobe 206, or a separate, dedicated RF sensor (such as a cavitationsensor) 208, in accordance with various embodiments of the invention.The MRI apparatus 200 includes RF transmitter coils 210 and gradientcoils 212 for generating time-varying magnetic gradients across thetissue to be imaged. Both transmitter-coil and gradient-coil emissionsfall in the RF range and can potentially disturb ultrasonictreatment/imaging procedures and/or cavitation detections (or otherconcurrently performed RF-sensitive operations). The MRI transmittercoils 210 generate electromagnetic pulses with frequencies in the rangefrom about 50 MHz to about 150 MHz to induce spin flipping. Thegradients generated by the gradient coils 212 are typically updated atkHz or MHz frequencies, and are substantially constant betweensuccessive updates. For example, the gradient value (i.e., the magneticfield strength of the gradient field) may be controlled digitally at asampling rate of 250 kHz by applying a new current every fourmicroseconds. These small control steps generate RF noise, mainly at thesampling frequency (i.e., 250 kHz in the example) and its harmonics(i.e., 500 kHz, 750 kHz, etc.). A step in the gradient value is usuallyimplemented by a controlled ramp whose slope is proportional to thecurrent step. The resulting RF noise is generally proportional to thecurrent step as well. However, even during nominally static gradients,control steps exist and resulting in some level of RF noise (althoughsignificantly less than is generated during ramps). In other words,non-zero static gradients are quieter than dynamic gradients, but arenot completely quiet.

Ultrasound imaging and measuring the cavitation spectrum of the acousticreflections generally have low associated signal voltages (e.g.,voltages in the 5 mV range and below). During these measurements, theultrasound receiver (which may be the transducer array 153 operated in a“listening” mode, or a separate, dedicated receiver device 172) and/orthe separate, dedicated RF sensors 208 may convert the acoustic signalsinto electrical RF signals. Such signals can also be created by the RFdisturbances from the MRI apparatus 200, resulting in unwanted signalcomponents. Since the detected signals generally have lower power than,e.g., focused ultrasound ablation pulses, they are particularlysensitive to such perturbations.

FIG. 3A shows a pulse sequence diagram illustrating, for a typical MRgradient echo pulse sequence, the relative timing of the RF excitationpulse 302, the magnetic field gradients 304, 306, 308 in threedirections, and the MR response signal 310, which occurs at the echotime (TE). The sequence may be periodically repeated; the period isdenoted as the repetition time TR and may be, for example, in the rangefrom 20 to 30 ms. FIG. 3 further shows the timing and the period duringwhich operations of the concurrently operated system may be carried out.In some embodiments, if operations of the concurrently operated systemare not especially sensitive to the RF disturbances (e.g., whenperforming focused ultrasound emissions for ablating the diseasedtissue), they can be carried out at any time, including the period 312during which the MRI gradients are active. If, however, the concurrentlyoperated system is RF-sensitive, the detections are carried out duringgradient idle times (or “quiet times”) 314 of the MRI system—i.e., timeintervals in which magnetic field gradients, or time variations thereof,are substantially unchanged and/or RF signals are not excited. In otherwords, operations of the MRI system 200 and the concurrently operatedsystem 202 are synchronized based on the MRI pulse sequence that iscomputed and controlled by the MR controller 214. For example, duringthe MR idle time 314, the MRI apparatus 200 may send a synchronizationsignal to a controller 216 of the concurrently operated system 202,which can then perform spectrum measurements and/or other RF-sensitiveoperations.

Alternatively, synchronization may be effected through external controlmechanisms. For example, referring again to FIG. 2, the controllermodule 216 of the concurrently operated system 202 and/or an externalcontroller 218 may control the timing of RF-sensitive operationsdirectly or via the MR controller 214 based on measurements of RFsignals originating from the MRI apparatus 200. For example, theexternal controller 218 may communicate with the MRI and concurrentlyoperated systems that each include individual controllers. The externalcontroller 218 may include a control module 222 that determines whengradient-field activity is quiet, e.g., based on information receivedabout an MRI pulse sequence specifying time intervals during which thegradients are quiet. The control module 222 may also send controlsignals to the MR controller 214 to stop MRI operation at the end of asequence in order to create quiet time. The control module 222 maycommunicate gradient idle time to a triggering module 224 responsiblefor initiating the RF-sensitive measurement operation. The controllers216, 218 are in communication with, for example, one or more RFreceivers (such as ultrasound channels) 220 associated with thephased-array ultrasound transducer system 204 or ultrasound imagingprobe 206 and/or by the separate, dedicated RF sensors 208 forperforming RF-sensitive signal measurements during the gradient idletimes 314. In some embodiments, the MR controller 214 and the controller216 of the concurrently operated system 202 may be integrated into asingle control module that sends synchronization or clock signalssimultaneously to both apparatus 200, 202, or controls the RFtransmitter coils 210, gradient coils 212, and concurrently operatedsystem 202 directly.

In various embodiments, the gradient idle times 314 of the MRI system200 are determined based on signals measured by the RF receiver(s) 220associated with the phased-array ultrasound transducer system 204 orultrasound imaging probe 206 and/or by the separate, dedicated RFsensor(s) 208. In one embodiment, the MRI system 200 is activated toperform standard operations while the concurrently operated system 202is idling—i.e., the concurrently operated system 202 is inactive or notperforming at least one of its functions (such as emitting acousticwaves) but is capable of detecting signals transmitted (this process isdenoted as a “cold” scan). Accordingly, the time periods during whichthe RF signals can be detected by the RF receiver(s) 220 and/or RFsensor(s) 208 are defined as gradient-active times, whereas the timeperiods during which the detected RF signals are below a threshold aredefined as gradient-idle times. The threshold may be a level that wouldinterfere with RF-sensitive operations. The gradient-active andgradient-idle times are defined against the period of the scan or arepeating portion thereof. Thus, because the pulse sequence isrecurring, the gradient-active and gradient-idle time intervals of theMRI pulse sequence may be “learned” based on the measurements of the RFreceiver(s) 220 and/or RF sensor(s) 208. In one embodiment, the learnedgradient-active and gradient-idle time intervals are stored in computermemory, which can be implemented as any type of volatile or non-volatile(e.g., Flash) memory. The activity of the concurrently operated system202 may then be synchronized to the MRI system 200 based on the storedMRI pulse sequence retrieved from the memory. This approach allows theRF-sensitive operations to be performed concurrently with the MRIscanning in a subsequent (or a new) MRI pulse sequence without the needto measure, in real time, RF signals originating from the MRI apparatus200 in order to identify the MRI gradient-active and gradient idle timeintervals each time RF-sensitive operations are to be performed.

The dedicated sensor(s) 208 may be made of a wire loop or a solenoidthat is sensitive to the MR electromagnetic interference but not to themeasuring signals originating from the concurrently operated system 202.The dedicated sensor(s) 208 may be placed inside the MR bore 106 in alocation that is sensitive to gradients along each axis but does notinterfere with the concurrently operated system 202 for performingRF-sensitive measurements during MR scans. Again, based on the measuredRF disturbance resulting from operation of the MRI apparatus 200, theMRI apparatus 200 and the concurrently operated system 202 aresynchronized so that the non-RF-sensitive operations are carried outduring gradient-active periods (or MR active periods), reserving thequiet times (or gradient-idle times) for RF-sensitive operations.

Referring to FIG. 3B, when more than one receiver 220 and/or sensor 208is simultaneously used to measure the RF signals, each receiver 220and/or sensor 208 may detect the RF disturbance with a differentamplitude and/or phase. In one embodiment, the MR gradient-activeperiods are defined as a sum (i.e., the union) of the active-gradienttimes detected by at least some of the individual receivers 220 and/orsensors 208. That is, if the MRI pulse sequence is not available to thecontroller 216, it can be learned.

In some embodiments, the MRI sequence stops running after it completes,and MR-sensitive operations are performed following the end of onesequence and before the beginning of the next sequence. The nextsequence may begin automatically or may be triggered by an externalcontrol signal. The concurrently operated system 202 identifies the endof a sequence (e.g., by measuring RF signals generated by the MRIapparatus using the dedicated receiver(s)/sensor(s)), performs theRF-sensitive measurements, and then sends a trigger command to the MRIapparatus indicating completion of the RF-sensitive measurements. TheMRI apparatus 200 may then execute the next sequence. This delay mayoccur between successive sequence or may, depending on the application,may be postponed or staggered so that the MRI sequence is repeated oneor more times before the RF-sensitive operation is carried out (i.e.,one or more sequence transitions is skipped). The system may also beprogrammed to perform the RF-sensitive measurements only after certainMRI procedures, for example, only after thermal imaging sequences.External control generally provides a high degree of flexibility intiming MR imaging and RF-sensitive measurements, thereby facilitatingtime efficiency in the overall procedure.

In general, functionality for synchronizing an MRI apparatus and aconcurrently operated system as described above, whether integrated withthe controllers of MRI and/or the concurrently operated system orprovided by a separate external controller, may be structured in one ormore modules implemented in hardware, software, or a combination ofboth. For embodiments in which the functions are provided as one or moresoftware programs, the programs may be written in any of a number ofhigh level languages such as FORTRAN, PASCAL™, JAVA™, C, C++, C#, BASIC,various scripting languages, and/or HTML. Additionally, the software canbe implemented in an assembly language directed to the microprocessorresident on a target computer; for example, the software may beimplemented in Intel™ 80x86 assembly language if it is configured to runon an IBM™ PC or PC clone. The software may be embodied on an article ofmanufacture including, but not limited to, a floppy disk, a jump drive,a hard disk, an optical disk, a magnetic tape, a programmable read-onlymemory (PROM), an erasable programmable read-only memory (EPROM),electrically erasable programmable read-only memory (EEPROM),field-programmable gate array, or CD-ROM. Embodiments using hardwarecircuitry may be implemented using, for example, one or morefield-programmable gate array (FPGA), complex programmable logic device(CPLD) or application-specific integrated circuit (ASIC) processors.

FIG. 4A illustrates a learning procedure that does not require a “cold”scan as described above. Instead, the concurrently operated system 202performs the RF-sensitive measurements during the MR gradient-activeperiods (i.e., “hot scans”); the measured signals 402 include the MRelectromagnetic interference. In one embodiment, while both the MRI andconcurrently operated systems are active during the hot scans, thededicated sensors 208 separately, and in parallel, detect the MRinterference 404. Because the dedicated sensors 208 are sensitive to theMR electromagnetic interference originating from MRI operations but notto the measured signals from the concurrently operated system 202,signals detected using the dedicated sensors 208 can be used to definethe MR gradient-active periods if these are not known to the controller216 in advance. As described above, the time periods during which thedetected RF signals exceed a threshold are defined as gradient-activetimes, whereas the time periods during which the detected RF signals arebelow a threshold are defined as gradient-idle times. Accordingly, theRF-sensitive measurements carried out using the concurrently operatedsystem 202 can be corrected by discarding the data measured during thetime intervals corresponding to active-gradient periods, which have beenlearned based on the measurements of the dedicated sensors 208.

In another embodiment, the signals measured by the concurrently operatedsystem 202 are corrected in accordance with a signal-correctionprocedure as further described below. FIGS. 4B and 4C depict asignal-correction procedure 410 that allows the MRI apparatus 200 andthe concurrently operated system 202 to operate simultaneously—i.e., theMRI apparatus 200 obtain MRI images and the concurrently operated system202 perform RF-sensitive measurements. In a first step 412, theconcurrently operated system 202 is first set in an idling mode—i.e., itis powered on but not activated to transmit energy. For example, if theconcurrently operated system 202 is a focused ultrasound system, noultrasound channels are actively driven to transmit acoustic waves yetthe ultrasound channels are ready to receive acoustic signals in theidling mode. In a second step 414, the MRI apparatus 200 is activatedand operated in accordance with a normal procedure—i.e., emitting apulse sequence of magnetic field gradients and RE excitation pulses intothe imaging region to cause the patient's tissues to emit MR responsesignals. As described above, this step is a “cold” scan as theconcurrently operated system is inactive during MR scanning. In a thirdstep 416, the MR interference 418 resulting from activations of the MRIapparatus 200 is measured by the detection channels and/or sensors(e.g., cavitation sensors) associated with the concurrently operatedsystem 202; the measured MR interference may be stored in computermemory, which be implemented as any type of volatile or non-volatile(e.g., Flash) memory. In a fourth step 420, both MRI apparatus 200 andconcurrently operated system 202 are simultaneously activated (i.e., a“hot” scan), while the detection channels and/or sensors receive signals422 transmitted thereto. Because the MRI interference 418 is typicallyan additive, stationary noise with respect to the measured signals,signals 422 detected by the concurrently operated system 202 can becorrected by subtracting the MRI interference measured during the coldscans from the detected signals 422 obtained during the hot scans (in afifth step 424), based on the temporal locations of the interfering MRpulses learned during the cold scan. This approach thus provideseffective corrections 426 to the desired signals measured by theconcurrently operated system 202 with no (or at least limited)MRI-caused interference.

Referring to FIG. 4D, the concurrently operated system 202 may beactivated with a period 428 shorter than the repetition time TR 430. Forexample, the cavitation measuring time may be in the order of 1 ms whilethe repetition time TR is in the range from 20 to 30 ms. Thus, thecavitation (or other) measurements 428 may not be synchronized with theTRs 430; successive measurement windows 432, 434 can occur duringdifferent intervals within each TR and/or successive TRs. In oneembodiment, to correct these cavitation signals (or other signals)measured during different time windows, a TR noise profile 436 is firstestablished using the cold scan as described above. Because the MR noisevaries throughout the TR duration, the TR noise profile 436 allows theoverall MR interference magnitude, which is a function of time duringthe TR, to be tracked. When the concurrently operated system 202 isactivated to perform cavitation (or other RF-sensitive) measurements 438during a hot scan, the concurrently operated controller 216 candetermine the time windows 440 a-440 g associated with the RF-sensitivemeasurements and subsequently identify the MR interference levels in theparticular measurement windows using the established TR noise profile436. Accordingly, the measured signal can be corrected by subtractingthe identified MR interference noise therefrom. Additionally, the MRinterference may be pseudo-stationary for each sequence repetition (ortime to repeat (TR)), so that only phases encoded in the gradientamplitudes are changed between the TRs. In some embodiments, the MRIinterferences obtained during the cold scans are reduced to a single setof data obtained from, for example, a single cold scan TR, an average ofMR interferences in multiple cold scan TRs, or a maximum MR interferencein the multiple cold scan TRs. This approach may advantageously reducethe complexity when processing a massive data volume of signal(s)acquired during simultaneous operations of the MRI apparatus andconcurrently operated system.

The signal-correction procedure as depicted in FIGS. 4B and 4C may becombined with the learning procedure of the MR gradient-active periodsas illustrated in FIG. 4A. For example, at the end of each MR sequence,the controller 216 of the concurrently operated system 202 may correctthe RF-sensitive signals measured during hot scans by discarding thedata measured during the active-gradient periods as described in FIG.4A. If, however, the discarded data is above a threshold (e.g., 30% ofthe acquired data), the controller 216 may start the signal-correctionprocedure—i.e., subtracting the MR interference measured during the coldscans from the measured RF-sensitive signals obtained during the hotscans—as described in FIGS. 4B and 4C or adjust the RF-sensitivemeasurements based on the measured magnitude of the MR interference inthe subsequent MR sequence(s). In another example, the controller 216may in real time subtract the MR interference from the receivedRF-sensitive signals. If, however, the controller 216 determines thatthe MR noise obtained during cold scans, is above a threshold that mayresult in unreliable RF-sensitive measurements, the controller 216 may,again, discard or reject the measured RF-sensitive signals during thehigh-noise time period(s).

Referring to FIG. 4E, in various embodiments, the MR interference noisemeasured during the cold scans and signals measured during the hot scansare converted into signals 448, 450, respectively, in the frequencydomain using, e.g., a Fast Fourier Transform (FFT) (steps 442 and 444,respectively, as depicted in FIG. 4B). Subtraction of the MRIinterference noise 448 from the measured signal(s) 450 can be thenperformed by directly subtracting the magnitude of the converted signalsat each frequency in the spectrum. This is particularly useful when themeasured signal(s) of the concurrently operated system are insensitiveto the phase measurements (e.g., cavitation measurements in focusedultrasound) and avoids the need to accurately synchronize the operationsof MRI scanning and the concurrently operated system.

In some embodiments, the synchronization and correction approachesdescribed above are used in conjunction with shielding, signalfiltering, and/or processing. For example, if the synchronizationapproach is combined with shielding, there is generally a trade-offbetween the amount of shielding used and the maximum acceptable noise.The less shielding is used, the quieter the gradients need to be toavoid undesired interference between the MRI system and the ultrasound(or other concurrently operated) system. If the signal correctionapproach is combined with shielding, the more shielding is used, theless correction is required. Noise reductions due to shielding depend onthe particular material used (e.g., iron, copper, or nickel) as well ason the frequency range of interest, and can readily be ascertained basedon graphs and tabulations of absorption and reflection coefficients thatare available in the literature. For example, at frequencies of around 1MHz, a 3 mm thick iron shield reduces the noise by about 100 dB. For agiven maximum acceptable noise level (which, in turn, depends on thesignal filtering and processing capabilities of the system), the maximumallowable gradients can be computed based on the noise reductionachieved by shielding.

Although the present invention has been described with reference to anultrasound transducer system and other specific details, it is notintended that such details should be regarded as limitations upon thescope of the invention. For example, systems and methods forsynchronizing MR imaging with treatment modalities other than focusedultrasound therapy that include RF-sensitive operations are alsoincluded within the scope of the invention. Moreover, the terms “MRinterference,” “MR interference noise,” “MR noise,” “RF noise,” and “RFdisturbance” are used herein interchangeably. Further, it is to beunderstood that the features of the various embodiments described hereinare not necessarily mutually exclusive and can exist in variouscombinations and permutations, even if such combinations or permutationsare not made express herein, without departing from the spirit and scopeof the invention. In fact, variations, modifications, and otherimplementations of what is described herein will occur to those ofordinary skill in the art without departing from the spirit and thescope of the invention.

What is claimed is:
 1. A method of performing magnetic resonance (MR)imaging of an anatomic region in conjunction with an ultrasoundmeasurement of the region using an ultrasound system, the methodcomprising the steps of: simultaneously performing an MR scan sequenceincluding MR pulses and the ultrasound measurement; recording theultrasound measurement as the ultrasound measurement is made; detectingtime intervals during the MR scan sequence when a radio frequency (RF)level is sufficient to interfere with the ultrasound measurement; anddiscarding the ultrasound measurement performed within the detected timeintervals and retaining only the ultrasound measurement performedoutside the detected time intervals.
 2. The method of claim 1, whereinthe detecting step is performed by the ultrasound system.
 3. The methodof claim 1, wherein the detecting step is performed by at least onededicated sensor outside the ultrasound system.
 4. A system forperforming magnetic resonance (MR) imaging of an anatomic region inconjunction with an ultrasound measurement of the region, the systemcomprising: an MR imaging apparatus for imaging the anatomic region; anultrasound system for performing the ultrasound measurement; and acontroller in communication with the MR imaging apparatus and theultrasound system, the controller being configured to: simultaneouslyperform an MR scan sequence including MR pulses and the ultrasoundmeasurement; record the ultrasound measurement as the ultrasoundmeasurement is made; determine time intervals during the MR scansequence when a radio frequency (RF) level is sufficient to interferewith the ultrasound measurement; and discard the ultrasound measurementperformed within the determined time intervals and retain only theultrasound measurement performed outside the determined time intervals.5. The system of claim 4, wherein the controller is implemented in theultrasound system and the determination of the intervals is performed bythe ultrasound system.
 6. The system of claim 4, wherein thedetermination of the intervals is performed by at least one dedicatedsensor outside the ultrasound system.
 7. The system of claim 4, whereinthe ultrasound system comprises at least one of a phased-arrayultrasound transducer system, an ultrasound imaging probe, or acavitation sensor.